1. Field of the Invention
This invention relates to magnetic seals for rotating shafts, and in particular, magnetic seals which minimize the presence of lubricants at the seal faces by using an improved drain-back design.
2. Background
Magnetic seals have proven reliable for use on rotary shafts. Magnetic seals have a stationary ring, referred to herein as a stator, and a rotating ring, referred to herein as a rotor. The stator is generally fixed to the shaft housing and does not rotate with the shaft. The rotor generally rotates with the shaft. The stator and rotor touch through sealing faces, which provides the seal between the stator and rotor. The sealing faces are subject to frictional forces due to the attractive forces of the magnets and the rotation of the rotor relative to the stator.
Various means are employed to maintain the seal between the sealing faces. Magnetic means are one method utilized to keep the stator and rotor contact faces in contact through the attractive forces of permanent magnets. The magnets can be positioned on either the stator or rotor, with the magnets interacting with ferrous materials in the opposite component. The features of magnetic seals are shown in U.S. Pat. No. 5,078,411 to Adams, hereby incorporated by reference. As is shown in Adams, the magnets can be positioned on either the stator (FIG. 7) or rotor (FIG. 4).
An improved magnetic seal is shown in U.S. Pat. No. 5,730,447 to Dawson. This patent discloses a “floating” inner annulus on the stator, which helps keep the contact faces in contact in the presence of shaft tilt or shaft misalignment. Another magnetic seal is shown in U.S. Pat. No. 6,805,358, also to Dawson, et al., which allows axial shaft movement in either direction without jeopardizing the sealing contact between the sealing faces.
Prior art magnetic rotary seals all attempt to prevent the seepage of lubricating oil across the sealing faces, because the oil must remain only within the machinery. Oil that travels outside of the proposed seal creates well known maintenance and environmental problems. Thus, the sealing faces are considered to the “primary seal”, or the main structural impediment to oil seeping outside of the rotating shaft. However, such attempts have met with limited success, because the even the best seal faces do not establish a perfect seal, especially in a vibrating and rotating environment. Therefore, it can be appreciated that in most prior designs which rely upon the seal faces as the primary seal, a common goal is to maintain a relatively high contact force between the sealing faces, under the theory that greater contact force will minimize oil seepage. However, some disadvantages to maintaining high contact forces are: (1) high operating temperatures due to increased friction at the sealing faces, (2) shorter operating life of the seal faces due to such friction, (3) use of stronger and more expensive magnets required to establish high contact forces, and (4) small operating gaps between the magnets and the opposing component in order to maintain high attractive forces.
The present design of an improved magnetic rotary seal discloses an advantageous structure that permits (and actually urges) lubricating oils which are drawn within the rotary seal to drain back out of the seal. Thus, if the oil is kept away from the rotating sealing faces, those sealing faces are no longer required to function as the “primary seal” as in prior designs. As will be seen below in the figures and description, this improved design resolves most of the problems mentioned above. For example, when the sealing faces no longer need to function as a primary seal, the high contact forces between them is no longer required. As a result, the seal operates with less friction and at lower temperatures, which substantially extends the life of the seal. Moreover, fewer or less powerful magnets may be required to establish the smaller contact force between the sealing faces. However, even with the same number or type of magnets, the operating gap between the magnets and the opposing component (the “magnetic operating gap”) can now be greater than prior designs. Greater magnetic operating gaps not only introduce flexibility and lower tolerances to the assembly, but they also provide a built-in allowance for gradual wear between the sealing faces before maintenance or refurbishment is required.